Aberrantly-active Stat3 is detected in primary, metastatic and drug resistant tumor phenotypes and is a master regulator of events that promote cancer development and progression. Substantive work from us and others has validated that Stat3-selective inhibitors would be therapeutically useful for a variety of human cancers. Initial proof-of-concept studies from us demonstrated strong antitumor effects of the lead Stat3 inhibitors, S3I-201 and S3I-201.1066 in mouse xenografts of human breast cancer. In the present study, BP-1-102 and other BP-series of agents derived from S3I-201.1066 were characterized for their biochemical, biological and antitumor activities and found to possess strong inhibitory activities against aberrant Stat3 activity, with binding affinity (KD) of 504 nM, and IC50 values of 0.7-15 ?M. In particular, BP-1- 102 and the newly-derived agents, XW-1-053 and SH-04-54 inhibited aberrant Stat3 DNA-binding activity outside and inside cells and induced antitumor cell effects against malignant cells harboring aberrant Stat3 activity at sub-micromolar (0.7-5 ?M) activities. Initial in vivo testing showed BP-1-102 strongly inhibit growth of human breast tumors in mouse xenografts. These very promising new chemical entities are among the most potent Stat3 inhibitors identified to date. Our objective is to pursue rigorous structural analyses of the leading agents' binding to Stat3 to derive structural data that will support a hypothesis-driven intensive medicinal chemistry effort to generate agents of optimum parameters for clinical development. We hypothesize that optimized analogs would potently inhibit Stat3 activity and functions in tumor cells, possess suitable in vivo bioavailability and have high antitumor efficacy in human tumor xenografts. To achieve the Aims and the Milestones, iterative rounds of optimization through chemistry, structural analysis, biochemistry/biology, and pharmacology efforts will be performed. Data from studies of the properties of agents in Aims 1 and 2 will be fed back into chemistry to guide the optimization and the cycle repeated until suitable agents are identified, which will be tested in Aim 3 for in vivo antitumor efficacy studies using human breast cancer models in mice. Our plan thereafter is to pursue advanced, pre-clinical IND enabling studies through the NCI's RAID program or to license out the 1-2 drug candidates to a pharmaceutical company for further pre-clinical development and eventually clinical testing. PUBLIC HEALTH RELEVANCE: The proposal seeks to derive clinical drug candidates from the BP-series of novel inhibitors of aberrantly-active Signal Transducer and Activator of Transcription (Stat)3, which have proven to inhibit the growth of human breast tumor xenografts in mice.